In recent years, OA equipment such as a computer, wordprocessor, and copying machine are becoming popular, and many printing apparatuses to be used for this equipment have been developed. Especially, inkjet printing apparatuses are superior to printing apparatuses employing other printing methods because of the easily attainable high resolution, high operation speed, quietness, and low cost. Recent OA equipment are required to be capable of color printing. To meet this requirement, a lot of color inkjet printing apparatuses have also been developed.
An inkjet printing apparatus discharges ink from nozzles to a printing medium, thereby forming an image. Particularly, to increase the print speed, a printhead with a plurality of ink orifices and liquid channels being integrated is used as a printhead in which a plurality of print elements are integrated and arrayed. A printing apparatus coping with color printing generally comprises a plurality of printheads (to be referred to as a multi-head hereinafter).
For color image printing, various factors such as color development, tonality, and uniformity must be taken into consideration, unlike monochrome printers that print only characters and numbers. Especially as for the uniformity, slight variations in nozzle to nozzle in the multi-head manufacturing process influence the amount and direction of ink discharge from each nozzle during color image printing. This finally appears as density unevenness in an image, resulting in a poor image quality.
A detailed example will be described with reference to the accompanying drawings.
FIG. 17 is a view showing an image density when ink is properly discharged. FIG. 18 is a view showing an image density when errors occur in an ink discharge amount and direction.
Referring to FIGS. 17 and 18, reference numeral 91 denotes a printhead; 92, an ink discharge nozzle (to be referred to as a nozzle hereinafter); 93, an ink droplet discharged from the nozzle 92; 94, a printing medium; and 95, a printed dot formed on the printing medium.
When all nozzles discharge ink droplets with the same size in the same direction, as indicated by a in FIG. 17, the printed dots 95 with the same size are formed on the printing medium 94, as indicated by b in FIG. 17. As a result, a uniform image without density unevenness is obtained as a whole, as indicated by c in FIG. 17.
In fact, the discharge amount and direction vary between the nozzles, as described above. Hence, if printing is executed without any correction, the size and discharge direction of the ink droplets 93 discharged from the nozzles 92 vary, as indicated by a in FIG. 18. Consequently, the printed dots 95 are formed on the printing medium 94 in different sizes or at unexpected positions, as indicated by b in FIG. 18. According to b in FIG. 18, a blank portion (a portion without printed dots) exists in the nozzle array direction, or conversely, the printed dots 95 overlap more than necessary to increase the printing density. Alternatively, a white stripe is formed, as can be seen at the center of b in FIG. 18. The set of printed dots formed in this manner shows a density distribution indicated by c in FIG. 18 in regard to the nozzle array direction.
As a result, the density variation is normally perceived as density unevenness by the human eye.
To solve the density unevenness, a method of executing divisional printing by repeatedly scanning a printhead in the same region of a printing medium and a method of executing divisional printing by disposing a plurality of printheads have been proposed conventionally.
As a head structure including a plurality of printheads, a so-called dual head structure in a serial printer and a structure having a so-called full-line printhead with a print width corresponding to the width of a printing medium in a line printer are known.
To achieve high-speed printing highly demanded recently, a line type inkjet printing apparatus is also known which comprises a full-line printhead having a print width equal to or more than the width of a printing medium and limits relative movement of the printhead and printing medium to one.
Full-line printheads include an “integrated line type” printhead having a full-line print width by one print element substrate on which nozzle arrays for discharging ink are arranged, and a “bonded-head line type” printhead which increases the print width by bonding a plurality of print element substrates with a short print width.
Even for the “bonded-head line type” printhead, many methods of arraying print element substrates are known. For example, print element substrates are arranged in a line at an interval to form one printhead. A region between the print element substrates where no printing is performed is printed by using another printhead. Alternatively, a printhead using a so-called “overlap” method is known in which print element substrates are arrayed to execute printing in the same region by the plurality of print element substrates provided on one printhead.
In a printhead which has an array of a plurality of print elements each having an ink orifice and an electrothermal transducer for generating discharge energy to discharge ink from the ink orifice, power required for driving these print elements is large. Hence, a time divisional driving method is widely known which divides a plurality of print elements into a plurality of blocks and sequentially drives the blocks (e.g., Japanese Patent Publication Laid-Open No. 8-72245).
According to this method, for example, a plurality of print elements are put into one block. Several or several ten driving integrated circuits each capable of simultaneously driving one print element in one block are arranged on a single substrate. Image data corresponding to the print elements is input, and the driving integrated circuits are time-divisionally driven, desired printing on a printing medium such as a printing paper sheet can be executed. In such time divisional driving, if adjacent print elements are driven simultaneously, the liquid channels mutually suffer pressure interference by pressure generated upon ink discharge. The printing density may change due to the pressure interference (crosstalk). Hence, it is desirable that simultaneous or continuous driving of adjacent print elements is inhibited, as is conventionally known.
To achieve high-quality printing by the conventional line type inkjet printing apparatus that implements high-speed printing, it is supposed to be effective to arrange a plurality of printheads and execute divisional printing by using the plurality of printheads. However, from the viewpoint of the cost, size, and power consumption of the printing apparatus, the number of the plurality of printheads is practically two or four at most.
Some of the conventional serial type inkjet printing apparatuses employ a multi-pass printing method using eight passes or more. It is difficult to implement an image quality equal to or better than that of the serial type by using a line type printhead.